1. Field of the Invention
The invention relates to a control apparatus and a control method for a hybrid vehicle having a secondary battery that can be charged with external power, or in other words a plug-in hybrid vehicle.
2. Description of Related Art
A hybrid vehicle that includes an internal combustion engine and a motor as vehicle-driving prime movers is available. The motor is either driven by the internal combustion engine or driven by vehicle inertia to function also as a power generator. Power generated thereby is stored in a storage apparatus installed in the vehicle, for example a secondary battery, and used to drive the motor. Further, when both a motor and a power generator are provided (as well as when the motor is caused to function as a power generator), the power generated by the power generator may be used as power for driving the motor without passing through the storage apparatus.
In recent years, a hybrid vehicle in which the storage apparatus installed in the vehicle can be charged with external power, or in other words a plug-in hybrid vehicle, has come into practical use. In a plug-in hybrid vehicle, control is performed such that at the beginning of a journey, the vehicle is basically driven by the motor alone, and when a storage amount of the storage apparatus decreases to a lower limit value, the vehicle is driven using the internal combustion engine concomitantly. Hereafter, a travel mode using only the motor at the beginning of a journey will be referred to as an electric vehicle mode, and a travel mode using the internal combustion engine and the motor concomitantly following a reduction in the storage amount will be referred to as a hybrid vehicle mode. Note, however, that when a required output exceeds an output of the motor in the electric vehicle mode, the internal combustion engine is started and a driving force thereof is also used. When the required output decreases, driving using only the motor is resumed.
In a plug-in hybrid vehicle, the electric vehicle mode is set at the beginning of a journey, and therefore opportunities to operate the internal combustion engine decrease. As a result, fuel consumption may not progress. In Japanese Patent Application Publication No. 2009-255680 (JP-2009-255680 A), when fuel remains in a fuel tank and a fuel pipe system for a long time, the fuel may deteriorate. When fuel deterioration is determined, the lower limit value of the storage amount is set to be higher, thereby increasing the number of opportunities to operate the internal combustion engine (see paragraph 0074 and FIGS. 7 and 10). By increasing the number of opportunities to operate the internal combustion engine, the fuel is consumed earlier, and therefore use of fuel in an advanced state of deterioration is suppressed.
Japanese Patent Application Publication No. 2012-030667 (JP-2012-030667 A) discloses control for increasing the number of opportunities to operate the internal combustion engine by increasing the lower limit value of the storage amount at which a switch to a driving mode using the internal combustion engine is determined when the fuel has deteriorated.
In JP-2009-255680 A and JP-2012-030667 A, the number of opportunities to operate the internal combustion engine is increased by increasing the lower limit value of the storage amount when fuel deterioration is determined, with the result that the fuel is consumed earlier. However, when a travel distance per journey is short, the storage amount does not decrease to the lower limit value, and therefore the mode is not switched. Hence, the internal combustion engine is not operated, and as a result, fuel consumption does not progress.